Silver nanowires, and production method and dispersion of the same

ABSTRACT

Silver nanowires coated with, instead of a polymer protective agent such as PVP, an organic protective agent having a smaller molecular weight are provided. The silver nanowires have an average diameter of 100 nm or less and an average length of 5 μm or more, and a thiol having a molecular weight of 75 to 300 is adhered on surfaces of the metal silver. The silver nanowires have a thiol containing one thiol group in the structure adhered thereon. A thiol having only one thiol group (—S—H) in a molecule is a suitable target. Examples thereof include 1-dodecanethiol, 1-decanethiol, 1-octanethiol, 3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalic acid.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to silver nanowires that are useful, forexample, as a material forming a transparent conductor, and a method forproducing the silver nanowires. The present invention also relates to asilver nanowire dispersion in which the silver nanowires are dispersed.

Description of the Related Art

As used herein, “nanowires” refers to an aggregate of fine metal wireshaving a thickness of approximately 200 nm or less. When likened topowder, an individual wire corresponds to a “particle” constitutingpowder, and nanowires correspond to “powder” which is an aggregate ofparticles.

Silver nanowires are expected as a promising conductive material forimparting electric conductivity to a transparent base material. When atransparent base material such as a glass, polyethylene terephthalate(PET), and polycarbonate (PC) is coated with a liquid in which silvernanowires are dispersed, and then the liquid component is removed, forexample, by evaporation, silver nanowires come in contact with eachother on the base material to thereby form a conductive network, andtherefore can realize a transparent conductor. In related arts, as thetransparent conductive material, metal oxide films typified by ITO havebeen mainly used for applications such as a transparent electrode.However, the metal oxide films have drawbacks such as high cost in filmformation and low bending resistance which is a factor inhibitingproduction of flexible end products. In addition, a conductive film fora touch panel sensor which is one of major applications of transparentconductors is required to have high transparency and high electricconductivity, and in addition, a demand for visibility becomesincreasingly higher now. In an ITO film in related arts, while thethickness of an ITO layer is required to be increased for enhancingelectric conductivity, the increase of the thickness leads todeterioration of transparency, failing to improve the visibility.

Silver nanowires are expected to overcome the above drawbacks inherentin the metal oxide films typified by ITO.

As a method for producing silver nanowires, a technique is known inwhich a silver compound is dissolved in a polyol solvent such asethylene glycol and metal silver is deposited in a linear shape byutilizing reducing ability of the solvent polyol in the presence of ahalogen compound, and polyvinylpyrrolidone (PVP) or a copolymer having avinylpyrrolidone constituting unit which is an organic protective agent(Patent Document 1-3, and Non-patent Document 1). PVP and the copolymerhaving a vinylpyrrolidone constituting unit are highly effectivesubstances as an organic protective agent for synthesizing silvernanowires in a high yield.

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: US 2005/0056118-   Patent Document 2: US 2008/0003130-   Patent Document 3: US 2014/0178247

Non-Patent Document

-   Non-patent Document 1: J. of Solid State Chem. 1992, 100, 272-280

SUMMARY OF THE INVENTION

Common silver nanowires coated with PVP in related arts are generallyprovided as a silver nanowire dispersion using an aqueous liquid mediumbecause of good dispersibility thereof in water and other polarsolvents. However, depending on the application, a silver nanowiredispersion using a solvent having small polarity or a nonpolarnon-aqueous solvent is in some cases desirably used. For responding sucha need, a technique in which the polymer protective agent such as PVPwhich has been needed for silver nanowires synthesis is substituted witha substance having good dispersibility in a liquid medium is effective.

The polymer protective agent such as PVP with which silver nanowires arecoated is desirably adhered in an amount as small as possible or notadhered at all from the viewpoint of improving electric conductivity dueto contacts between the wires. However, since one molecule of thepolymer protective agent such as PVP adheres to metal silver of thewires at multiple points, it is generally difficult to effectivelyremove the polymer protective agent through a washing operation. It isalso difficult to substitute the polymer protective agent with anothersurface protective agent.

On the other hand, in formation of a conductive network by silvernanowires, the conduction has been secured by simple contacts betweenthe wires in related arts. If wires can be bonded at the contact pointsby sintering, the electric conductivity of a conductive network isconsidered to be greatly enhanced. However, silver nanowires having thepolymer protective agent such as PVP adheres thereon have a highsintering temperature, and it is difficult in many cases to sinter thesilver nanowires after applying them on a transparent base material. Inparticular, PVP also has a problem of easy fusing under heat.

The present invention has an object to provide silver nanowires coatedwith, instead of a polymer protective agent such as PVP, an organicprotective agent having a smaller molecular weight.

As a result of studies, the present inventors have found an effectivetechnique for securely substituting a polymer protective agent such asPVP with a substance having a smaller molecular weight with an easyprocedure using, as raw material wires, silver nanowires obtained by amethod in which silver is reduced and deposited in a wire form in analcohol solvent in the presence of the polymer protective agent.Specifically, the above object can be achieved by adopting a thiol asthe protective agent substance that is the substitute.

That is, in the present invention, silver nanowires having an averagediameter of 100 nm or less and an average length of 5 μm or more inwhich a thiol having a molecular weight of 75 to 300 is adhered onsurfaces of metal silver are provided. Silver nanowires having anaverage diameter of 50 nm or less and an average length of 10 μm or moreare a preferred target. Silver nanowires having an average aspect ratioof 250 or more are a particularly suitable target. As used herein, theaverage diameter, the average length, and the average aspect ratio aredefined as follows.

Average Diameter

On a microscopic image (for example, FE-SEM image), diameters of circlesinscribed in the opposite outlines in the thickness direction on aprojection image of a metal wire are measured along the entire length ofthe wire, and the average of the diameters is defined as the diameter ofthe wire. Then, the average of diameters of wires constituting nanowiresis defined as the average diameter of the nanowires. For calculating theaverage diameter, the total number of the wires to be measured is 100 ormore. However, wire-like products having a length (described later) lessthan 0.5 μm and particulate products are excluded from the wires to bemeasured.

Average Length

On the same microscopic image as above, a length of a metal wire fromone end to the other end along a line passing through positions of thethickness centers of the wire (that is, the centers of inscribed circlesas described above) on a projection image of the wire is defined as thelength of the wire. Then, the average of lengths of wires constitutingnanowires is defined as the average length of the nanowires. Forcalculating the average length, the total number of the wires to bemeasured is 100 or more. However, wire-like products having a lengthless than 0.5 μm and particulate products are excluded from the wires tobe measured.

The silver nanowires according to the present invention are constitutedof wires of a highly elongated shape. For this reason, the silvernanowires aggregated do not have a linear rod shape, but rather have astring-like curved shape in many cases. The present inventors createsoftware for measuring a length of such a wire having a curved shape inan efficient manner on an image and use the software in data processing.

Average Aspect Ratio

The average aspect ratio is calculated by assign the average diameterand the average length described above in the following formula (1).

[Average aspect ratio]=[Average length (nm)]/[Average diameter(nm)]  (1)

As the thiol, a compound having only one thiol group (—S—H) in amolecule is suitable. Examples include 1-dodecanethiol (molecular weight202.4), 1-decanethiol (molecular weight 174.4), 1-octanethiol (molecularweight 146.3), 3-mercapto-1,2-propanediol (molecular weight 108.2),monoethanolamine thioglycolate (molecular weight 153.2), ammoniumthioglycolate (molecular weight 109.1), and thiomalic acid (molecularweight 150.2).

The thiol to be adhered is not limited to one kind alone, and two ormore thiols may be adhered. The position of the thiol group present inthe thiol is not limited to the terminal end, and the thiol group mayexist at any position in a thiol molecular structure.

As specific aspects of the silver nanowires, the following (a) to (e)may be mentioned.

(a) Silver nanowires having an average diameter of 100 nm or less and anaverage length of 5 μm or more, wherein a thiol having a molecularweight of 75 to 300 is adhered on surfaces of metal silver.(b) The silver nanowires according to the above (a), which has anaverage diameter of 50 nm or less and an average length of 10 μm ormore.(c) The silver nanowires according to the above (a) or (b), wherein thethiol has only one thiol group in a molecule.(d) The silver nanowires according to the above (a) or (b), wherein thethiol is one or more selected from the group consisting ofi-dodecanethiol, 1-decanethiol, and 1-octanethiol.(e) The silver nanowires according to the above (a) or (b), wherein thethiol is one or more selected from the group consisting of3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammoniumthioglycolate, and thiomalic acid.

As a method for producing the silver nanowires having a thiol adheredthereon, there is provided a production method comprising mixing in acontainer a liquid A in which silver nanowires coated with a polymer isdispersed and a liquid B in which a thiol having a molecular weight of75 to 300 is dissolved, thereby substituting an adhering substance onmetal silver surfaces from the polymer to the thiol.

Examples of the polymer include polyvinylpyrrolidone (PVP), a copolymerhaving a vinylpyrrolidone constituting unit, and a polymer in whichpolymerizable monomers including acrylamide are polymerized. Thecopolymer having a vinylpyrrolidone constituting unit is a copolymerhaving a polymerized composition of vinylpyrrolidone and anothermonomer. Examples may include a copolymer of vinylpyrrolidone anddiallyldimethylammonium nitrate, a copolymer of vinylpyrrolidone and amaleimide-based monomer, and a copolymer of vinylpyrrolidone and acopolymer of acrylate monomers such as ethyl acrylate and hydroxyethylacrylate. The weight average molecular weight of the polymer used is,for example, 20,000 to 1,300,000.

The amount of the polymer adhered on silver nanowires in the liquid A(that is, silver nanowires after a synthetic reaction and washingtreatment of the silver nanowires) is, for example, 3 to 15% by mass,based on the ignition loss in TG-DTA (thermogravimetry-differentialthermal analysis) of dried silver nanowires. When the polymer onsurfaces of the silver nanowires is substituted with a thiol having alower molecular weight, the ignition loss in the TG-DTA is reduced ascompared with the value before the substitution treatment. As the thiolin the liquid B, the compounds mentioned above are exemplified.

The liquid A and the liquid B can be mixed in the presence of anamphipathic substance such as acetone and isopropanol. In particular,when a solvent for the liquid A is a polar solvent and a solvent for theliquid B is a nonpolar solvent, the mixing under the presence of anamphipathic substance is highly effective.

As specific aspects of the method for producing silver nanowires, thefollowing (f) to (n) may be mentioned.

(f) A method for producing silver nanowires comprising mixing a liquid Ain which silver nanowires coated with a polymer are dispersed and aliquid B in which a thiol having a molecular weight of 75 to 300 isdissolved to thereby substitute an adhering substance on metal silversurfaces from the polymer to the thiol.(g) The method for producing silver nanowires according to the above(f), wherein the silver nanowires dispersed in the liquid A have anaverage diameter of 100 nm or less and an average length of 5 μM ormore.(h) The method for producing silver nanowires according to the above (f)or (g), wherein the silver nanowires dispersed in the liquid A have anaverage diameter of 50 nm or less and an average length of 10 μm ormore.(i) The method for producing silver nanowires according to any one ofthe above (f) to (h), wherein the thiol has only one thiol group in amolecule.(j) The method for producing silver nanowires according to any one ofthe above (f) to (h), wherein the thiol is one or more selected from thegroup consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.(k) The method for producing silver nanowires according to any one ofthe above (f) to (h), wherein the thiol is one or more selected from thegroup consisting of 3-mercapto-1,2-propanediol, monoethanolaminethioglycolate, ammonium thioglycolate, and thiomalic acid.(l) The method for producing silver nanowires according to any one ofthe above (f) to (k), wherein the polymer is polyvinylpyrrolidone (PVP).(m) The method for producing silver nanowires according to any one ofthe above (f) to (k), wherein the polymer is a copolymer having avinylpyrrolidone constituting unit.(n) The method for producing silver nanowires according to any one ofthe above (f) to (m), wherein the liquid A and the liquid B are mixed inthe presence of one or two of acetone and isopropanol.

The present invention also provides a silver nanowire dispersion inwhich the silver nanowires having a thiol adhered thereon are dispersedin a liquid medium. As a specific aspect, a silver nanowire dispersionin which the silver nanowires as set forth in any one of the above (a)to (e) are dispersed in a liquid medium may be mentioned.

In the present invention, silver nanowires having a thiol adhered onsurfaces of metal silver are disclosed. On the silver nanowiresimmediately after synthesis through reduction and deposition, an organicsubstance such as a polymer used in the synthesis as a shape controlleris adhered. Since thiols have high adhesiveness to metal silver, when asurfactant containing a thiol is added to the silver nanowire dispersionafter the synthesis, the organic substance such as a polymer adhered onthe surfaces of the silver nanowires can be substituted with thesurfactant containing the thiol. By substituting an organic protectiveagent adhered on the silver nanowire surfaces, it becomes possible todisperse silver nanowires, which can generally be dispersed only in apolar solvent, in a nonpolar solvent such as toluene and hexane.

Monomers having a molecular weight of 300 or less are easily releasedfrom surfaces of silver nanowires at a lower temperature compared with apolymer such as PVP. For this reason, when a transparent conductive filmis formed using the silver nanowires according to the present invention,through a heat treatment that is performed after a coating materialcontaining the silver nanowires is applied on a base material, theamount of organic substances remaining on the surfaces of the nanowirescan be reduced as compared with related arts. A smaller amount oforganic substances on surfaces of nanowires leads to increase ofprobability of contacts between the nanowires and decrease of electricresistance of the conductive network. Since the density of silvernanowires in a film required for imparting a prescribed electricconductivity to a conductive film can be reduced, optical transparencyis accordingly enhanced, being advantageous for forming a clearconductive film with less haze.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a structural formula of 1-dodecanethiol.

FIG. 2 shows a structural formula of 1-decanethiol.

FIG. 3 shows a structural formula of 1-octanethiol.

FIG. 4 shows a structural formula of 3-mercapto-1,2-propanediol.

FIG. 5 shows a structural formula of monoethanolamine thioglycolate.

FIG. 6 shows a structural formula of ammonium thioglycolate.

FIG. 7 shows a structural formula of thiomalic acid.

FIG. 8 is an SEM photograph of polymer (PVP)-coated silver nanowiressynthesized in Example 1.

FIG. 9A is an SEM photograph of the polymer-coated silver nanowiresbefore coating substance substitution treatment used in Example 1.

FIG. 9E is an SEM photograph of thiol-coated silver nanowires aftercoating substance substitution treatment obtained in Example 1.

FIG. 10 shows X-ray diffraction patterns of the silver nanowires beforeand after coating substance substitution in Example 1.

FIG. 11 shows TG curves of the silver nanowires before and after coatingsubstance substitution in Example 1.

FIG. 12 is an SEM photograph of polymer (copolymer of vinylpyrrolidoneand diallyldimethylammonium nitrate)-coated silver nanowires synthesizedin Example 4.

FIG. 13A is an SEM photograph of the polymer-coated silver nanowiresbefore coating substance substitution treatment used in Example 4.

FIG. 13B is an SEM photograph of thiol-coated silver nanowires aftercoating substance substitution treatment obtained in Example 4.

FIG. 14 shows TG curves of the silver nanowires before and after coatingsubstance substitution in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

A thiol is a substance represented by the general formula R—SH. A thiolgroup (—S—H) has a nature of strongly binding to a noble metal such asgold and silver. When a thiol is added to a dispersion of silvernanowires having a surfactant such as PVP adhered thereon, the thiolhaving a stronger adhesiveness with silver tends to preferentiallyadhere to wire surfaces. It has been found that by stirring the liquidor mixing the liquid with inversion to increase chances of the thiolgetting close to the metal silver surfaces of the wires, it is possibleto substitute the surfactant (organic protective agent) on the wiresurfaces with the thiol. Some surfactants such as amines and carboxylicacids do not have capability of substituting a high molecular surfactantsuch as PVP.

The thiol for use in the present invention suitably has a molecularweight in a range of 75 to 300. Within this range, the thiol is easy tobe released in a heat treatment in production of a conductive film evenat a relatively lower temperature, which is advantageous for obtaining atransparent conductive film with high electric conductivity.

In addition, it is preferred that a thiol having only one thiol group ina molecule is adopted. Since a thiol group has strong binding force tosilver, when two or more thiol groups are contained in a molecule, therespective thiol groups may adhere to different wires. In such a case,the individual wires are liable to bond to each other to formagglomerate. In order to disperse the individual wires independently ina liquid, use of a thiol having only one thiol group in a molecule iseffective.

Among thiols, there are ones that can dissolve in a nonpolar solvent andones that can dissolve in a polar solvent. A thiol providing asurfactant action according to the purpose may be selected. Examples ofthe thiol dissolved in a nonpolar solvent include 1-dodecanethiol,1-decanethiol, and 1-octanethiol. Examples of the thiol dissolved in apolar solvent include 3-mercapto-1,2-propanediol, monoethanolaminethioglycolate, ammonium thioglycolate, and thiomalic acid. As usedherein, a nonpolar solvent refers to a solvent having a solubilityparameter (SP value) of 9.5 or less. For example, water, methanol,ethanol, etc. fall under the polar solvent, and hexane, toluene,benzene, etc. fall under the nonpolar solvent.

When the amount of the thiol adhered to silver nanowires is excessivelylarge, the amount of thiol not leaving the wires but remaining on thewires in the heat treatment in production of a conductive film isincreased, and the electric conductivity of the conductive film may notbe sufficiently enhanced. According to a result of various studies, theamount of the thiol adhered is preferably 10.0% by mass or less, andmore preferably 5.0% by mass or less, relative to the total mass of thesilver nanowires (the sum of the metal silver and the thiol adhered tothe surfaces thereof). On the other hand, when the amount of the thioladhered is too small, there may arise problems of reduced dispersibilityin the liquid and agglomeration of silver nanowires. The amount of thioladhered is preferably 0.5% by mass or more. The amount of the thioladhered can be determined by an ignition loss obtained from TG-DTA(thermogravimetry-differential thermal analysis).

As for the shape of the silver nanowires, the nanowires desirably has anaverage diameter of 100 nm or less and an average length of 5 μm ormore, more preferably has an average diameter of 50 nm or less and anaverage length of 10 μm or more. A smaller average diameter leads to asmaller scattering of light in the transparent conductive film andhigher optical transparency, and therefore is advantageous for producinga clear and highly visible touch panel or display having less haze. Alarger average length leads to increased chances of contact betweenwires in forming a conductive network and therefore is advantageous forenhancing the electric conductivity of the transparent conductive film,accordingly making it possible to reduce the content of wires in theconductive film. The reduction of the wire content is effective forreducing the haze and enhancing the optical transparency. However, whenan average diameter is significantly reduced, fracture (break, rupture,etc.) of the wires is liable to occur in the production process ofconductive film, and therefore careful handling is required, which maylead to reduction in productivity. In general, an average diameter maybe 10 nm or more. In addition, when the average length is significantlyincreased, there may arise problems that the wires are entangled witheach other to agglomerate in a dispersion, or are liable to clog anozzle in application of a silver nanowire ink. The average length maygenerally be in a range of 500 μm or less. The average aspect ratio ismore preferably 300 or more.

The synthetic method of metal nanowires is not particularly limited, buta synthetic method by a wet process is known at this time. For example,in the case of silver nanowires, the reduction and deposition methoddisclosed in Patent Documents 1 and 2 (mentioned herein above) is known.

An operation for substitute an organic protective agent adhered on thesynthesized silver nanowires from a high molecular compound such as PVPto a surfactant having a thiol group which is a target substance isconducted. The silver nanowires of the present invention arecharacterized in that a surfactant having a thiol group is adhered.Since thiols has a nature of easily adhering to silver, when asufficient amount of a thiol exists in the vicinity of surfaces ofsilver nanowires having a high molecular compound such as PVP adheredthereon, there arises a state where the high molecular compound tends toleave the silver nanowire surfaces and meanwhile the thiol tends toadhere, and the substitution reaction can proceed relatively easily.

Since the reaction proceeds in a solvent, the thiol has to be dissolvedin a solvent. When the substitution is made with 1-dodecanethiol,1-decanethiol, 1-octanethiol, and the like which can be dissolved in anonpolar solvent, the liquid B described later is prepared using anonpolar solvent such as hexane and toluene. One kind of the nonpolarsolvents may be used alone or two or more kinds thereof may be used inmixture. In the case where the substitution is made with3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammoniumthioglycolate, thiomalic acid, and the like which can be dissolved in apolar solvent, the liquid B described later is prepared using a polarsolvent such as water, methanol, and ethanol. One kind of the polarsolvents may be used alone or two or more kinds thereof may be used inmixture.

When the amount of the thiol used is 0.5 mmol or more relative to 10 mgof the silver mass in the silver nanowires, the reaction (desorption andadsorption) for substituting a polymer adhered to the wire surfaces withthe thiol can sufficiently proceed. When the amount of the thiol islower, the substitution does not proceed sufficiently. For example, inthe case where a polar solvent in which silver nanowires having PVPadhered thereon is dispersed and a nonpolar solvent (for example,toluene) in which 1-dodecanethiol is dissolved are mixed to conduct anoperation for substituting an organic protective agent from PVP to1-dodecanethiol, when the amount of 1-dodecanethiol is sufficient,almost all the amount of PVP on the wire surfaces is substituted with1-dodecanethiol, and all the silver nanowires lose dispersibility in apolar solvent and meanwhile acquire dispersibility in a nonpolarsolvent. As a result, all the amount of the silver nanowires becomesdispersed in a nonpolar solvent layer among two solvent layers existingin a separated state. However, when the amount of 1-dodecanethiol addedis short, not all the amount of PVP can be substituted with1-dodecanethiol, wires still having PVP adhered on a part or all of thesurface remain in a polar solvent layer among the two layers separated.

In the operation of substituting an organic protective agent adhered onmetal silver surfaces of silver nanowires from a polymer to a thiol, aliquid A in which silver nanowires coated with the polymer are dispersedand a liquid B in which a thiol having a molecular weight of 75 to 300is dissolved are prepared. A polymer such as PVP which is used inreductive synthesis of silver nanowires generally has gooddispersibility in water. The silver nanowires coated with the polymerare stored in a state of being dispersed in an aqueous solvent in manycases. The aqueous solvent is a polar solvent.

In the case where the polymer is substituted with a thiol havingsolubility in a polar solvent, since the liquid B is a polar solvent, byusing as the liquid A the aqueous solvent in which silver nanowirescoated with the polymer is dispersed, the silver nanowire dispersion inwhich silver nanowires coated with the thiol is dispersed in a polarsolvent (an aqueous solvent, etc.) can be obtained.

On the other hand, in the case where the polymer is substituted with athiol having solubility in a nonpolar solvent, since the liquid B is anonpolar solvent, when an aqueous solvent in which silver nanowirescoated with the polymer are dispersed is used as it is as the liquid A,the liquid A and the liquid B separate with an interface formed. Forcausing adsorption of the thiol on wire surfaces of metal silver,molecules of the thiol have to come sufficiently close to the wiresurfaces present in the liquid A. When the liquid A and the liquid Bwhich are separated from each other are stirred vigorously, the chancesof the wire surfaces and the thiol molecules getting close to each otherare secured, but the nanowires are liable to suffer impact of thestirring to be damaged. Thus, in order to easily secure the chances ofthe wire surfaces and the thiol molecules getting close to each other ina stirring environment as mild as possible, it is effective that theliquid A and the liquid B are mixed in the presence of an amphipathicsubstance such as acetone and isopropanol. By adding the amphipathicsubstance into the liquid A or the liquid B in advance or placing theamphipathic substance in a container for mixing the liquid A and theliquid B in advance, the mixing in the presence of the amphipathicsubstance is possible. Acetone and isopropanol have solubilityparameters (SP values) exceeding 9.5, and therefore are not classifiedinto a nonpolar solvent referred to herein, but have affinity to both ofa polar solvent and a nonpolar solvent. In the case where the liquid Aor the liquid B having such an amphipathic substance added is used,chances of the wire surfaces and the thiol molecules getting close toeach other are easily secured when both the solvents are mixed andstirred, the substitution of the polymer with the thiol can proceedsmoothly. In addition, the wires after the substitution with the thiolcan easily transfer into the nonpolar solvent layer supplied from theliquid B. In this manner, a silver nanowire dispersion in which silvernanowires coated with a thiol are dispersed in a nonpolar solvent(hexane, toluene, benzene, etc.) can be obtained.

When an example where acetone or isopropanol is added to an aqueoussolvent in which silver nanowires coated with a polymer are dispersed toprepare the liquid A is mentioned, the amount of acetone or isopropanoladded may be 0.5 to 2.5 volume units based on 1 volume unit of theaqueous solvent.

Incidentally, in the case where the dispersion of the silver nanowirescoated with a polymer is an alcohol-based polar solvent such as methanoland ethanol, interfacial energy between the alcohol and a nonpolarsolvent is significantly smaller compared with interfacial energybetween water and the nonpolar solvent. For this reason, also when suchan alcohol-based solvent as above is used as it is as the A liquidwithout addition of any amphipathic substance, the substitution with athiol having dispersibility into a nonpolar solvent can be achievedrelatively easily.

The treatment temperature upon substituting an organic protective agentis a temperature of the boiling point of the solvent used or lower.Since the substitution reaction (desorption of a polymer and adsorptionof a thiol) proceeds easily even in normal temperature, the reaction maybe performed generally at a temperature in a range of 20 to 50° C. Asfor the method of mixing the liquid A and the liquid B, it is preferredthat the liquid B is added to the liquid A containing silver nanowires.The liquid B may be added at once or may be added intermittently orcontinuously. The liquid A is preferably brought into a state in advancewhere the liquid A is stirred at a speed that gives as small as possibledamage on the nanowires. The mixture may be stirred or mixed withinversion after the whole volume of the liquid B is added. Theatmosphere of the gas phase in contact with the liquid surface of thesolvent upon the substitution treatment is not particularly limited. Anair atmosphere, a nitrogen atmosphere or the like may be applied. Thetime required for the substitution reaction with a thiol is, in the caseof substitution at 20 to 50° C., from about 10 seconds to about 2minutes from the start of the mixing of the liquid A and the liquid B.

When a thiol having a carbon chain such as 1-dodecanethiol,1-decanethiol, and 1-octanethiol is adhered, the silver nanowires afterthe substitution treatment with the thiol become able to be dispersed ina nonpolar solvent such as hexane and toluene. When a thiol having ahydroxy group or a carboxy group, such as 3-mercapto-1,2-propanediol,monoethanolamine thioglycolate, ammonium thioglycolate and thiomalicacid is adhered, the silver nanowires can be dispersed in a polarsolvent such as water, methanol, and ethanol. In this case,dispersibility to various polar solvents can be controlled depending onthe kind of the thiol adhered.

EXAMPLES Example 1 Synthesis of Silver Nanowires

Polyvinylpyrrolidone (PVP) having a weight average molecular weight of55,000 was provided as an organic protective agent.

At normal temperature, 2.5 g of PVP and 0.006 g (0.1 mmol) of sodiumchloride were added to and dissolved in 60 g of ethylene glycol toprepare a solution X. In a container different from the above, 0.85 g(5.0 mmol) of silver nitrate was added to and dissolved in 7.65 g ofethylene glycol to prepare a solution Y.

Under an air atmosphere, the whole volume of the solution X was heatedto 135° C. with stirring at 500 rpm, and then the solution Y was addedat once into the solution X. After completing the addition of thesolution Y, the stirring speed was changed to 100 rpm, and the solutionwas maintained at 135° C. for 3 hours while keeping the stirring state.Then, the reaction liquid was cooled to normal temperature.

After cooling, washing was performed according to the followingprocedure to obtain a dispersion of silver nanowires.

The reaction liquid was transferred into a centrifugal tube, 30 mL ofdistilled water was added thereto, and the mixture was centrifuged underconditions of a centrifugal force of 1000 G and 5 minutes. After thecentrifugation, the supernatant was removed to collect a solid. After 30mL of methanol was added to the solid collected to disperse the solidtherein, the dispersion was centrifuged under conditions of acentrifugal force of 700 G and 5 minutes. After the centrifugation, thesupernatant was removed to collect a solid. After 30 mL of methanol wasadded to the solid collected to disperse the solid therein, thedispersion was centrifuged under conditions of a centrifugal force of250 G and 10 minutes. The supernatant was removed, and the solid wasdispersed again in water, thereby obtaining an aqueous dispersion of thesilver nanowires.

A sample was taken from the dispersion, the solvent water wasvolatilized on an observation stand, and then the sample was observed byFE-SEM (field emission type scanning electron microscope). As a result,the solid was confirmed to be silver nanowires. In this manner, silvernanowires coated with PVP were obtained.

In FIG. 8, an SEM photograph of silver nanowires is illustrated. In theSEM observation, in five fields selected randomly, the average diameterand the average length were determined according to the definitiondescribed herein above. The total number of the wires to be measured was100 or more. The wire diameters were measured on an image taken at amagnification of 150,000, and the wire lengths were measured on an imagetaken at a magnification of 2,500.

According to the result, the average diameter was 65.5 nm, the averagelength was 11.4 μm, and the average aspect ratio was 11400 nm/65.5nm≅174.

Substitution of Silver Nanowire Coating Substance with Thiol

As a thiol for substitution on the wire surfaces, 1-dodecanethiol havingsolubility in a nonpolar solvent was provided.

The aqueous dispersion of the PVP-coated silver nanowires synthesized bythe above method was adjusted in concentration so as to give a silverconcentration of 0.5% by mass. At a normal temperature, 5 mL of acetonewas added to 10 mL of the silver nanowire dispersion, and the mixturewas stirred for 10 seconds. The resulting mixture was a liquid A. Usinga container different from the above, 1-dodecanethiol was added to 10 mLof hexane which is a nonpolar solvent so as to give a concentration of10 mmol, and the mixture was centrifuged for 1 minute. The resultingmixture was a liquid B. The liquid B was added to the liquid A, andafter confirming separation into two layers, stirring was performed for1 minute. The silver nanowires were dispersed in a hexane solvent layeroriginating in the liquid B, and an aqueous solvent layer originating inthe liquid A became colorless and transparent. The lower aqueous solventlayer was drawn out and removed, the hexane solvent layer in which thesilver nanowires were dispersed was collected and transferred into acentrifugal tube, 30 mL of isopropanol was added thereto, and themixture was centrifuged under conditions of a centrifugal force of 250 Gand 10 minutes. After the centrifugation, the supernatant was removed tocollect a solid. After 30 mL of toluene was added to the solid collectedto disperse the solid therein, the dispersion was centrifuged underconditions of a centrifugal force of 250 G and 10 minutes. Thisoperation was repeated twice, and the obtained final solid was dispersedin toluene, whereby a nonpolar solvent (toluene) dispersion of silvernanowires was obtained.

In FIG. 9A, an SEM photograph of the polymer-coated silver nanowiresbefore the coating substance substitution treatment (the silvernanowires of FIG. 8 above observed under the same conditions as in FIG.9B below) is illustrated. In FIG. 9B, an SEM photograph of thethiol-coated silver nanowires after the coating substance substitutiontreatment is illustrated.

In FIG. 10, X-ray diffraction patterns by Cu-Kα ray of the silvernanowires before and after the coating substance substitution are shown.Both before and after the coating substance substitution, an X-raydiffraction pattern of metal silver is given.

In each of the metal nanowires before the coating substance substitutionand those after the substitution, the liquid component was volatilizedin a drying oven to obtain a dry sample. Using a TG-DTA apparatus(manufactured by Rigaku Corporation, Thermo Plus TG-8120), 10 mg of thedry sample was heated from a normal temperature to 600° C. at atemperature rising rate of 5° C./min, and the weight of the sample ateach temperature was measured every 1 second, thereby obtaining a TGcurve.

In FIG. 11, TG curves of the dry samples of the silver nanowires beforeand after the coating substance substitution are illustrated. When theignition losses until T=600° C. were compared, with an ignition lossuntil T° C. being represented by ((sample weight at temperatureT)−(sample weight before temperature rising))/(sample weight beforetemperature rising)×100, the polymer (PVP)-coated silver nanowiresbefore the coating substance substitution had a value of 3.8%, and thethiol (1-dodecanethiol)-coated silver nanowires after the coatingsubstance substitution had a value of 1.2%. In this example, silvernanowires coated with an organic protective agent (surfactant) havinggood dispersibility in a nonpolar solvent could be obtained. Since thesurfactant is a thiol having only one thiol group in a molecule, unlikea polymer having strong multiple-point adsorptivity such as PVP, thesurfactant is considered to be adsorbed on a metal silver surface of awire at one point in the molecule.

Example 2

A nonpolar solvent dispersion of silver nanowires was obtained in thesame manner as in Example 1 except that in the substitution operation ofsilver nanowire coating substance into a thiol, 1-decanethiol was usedas the thiol for substitution on the wire surfaces. As the silvernanowires subjected to the substitution operation, the same nanowires asin Example 1 (PVP-coated silver nanowires) were used. According to a TGcurve obtained under the same conditions as in Example 1, the ignitionloss until 600° C. of the thiol (1-decanethiol)-coated silver nanowiresafter the coating substance substitution obtained in this example was2.2%.

Example 3

A nonpolar solvent dispersion of silver nanowires was obtained in thesame manner as in Example 1 except that in the substitution operation ofsilver nanowire coating substance into a thiol, 1-octanethiol was usedas the thiol for substitution on the wire surfaces. As the silvernanowires subjected to the substitution operation, the same nanowires asin Example 1 (PVP-coated silver nanowires) were used. According to a TGcurve obtained under the same conditions as in Example 1, the ignitionloss until 600° C. of the thiol (1-octanethiol)-coated silver nanowiresafter the coating substance substitution obtained in this example was1.2%.

Example 4 Synthesis of Silver Nanowires

As an organic protective agent, a copolymer of vinylpyrrolidone anddiallyldimethylammonium nitrate (the copolymer was produced from 99% bymass of vinylpyrrolidone and 1% by mass of diallyldimethylammoniumnitrate, weight average molecular weight: 130,000) was provided.

At a normal temperature, 5.24 g of the copolymer of vinylpyrrolidone anddiallyldimethylammonium nitrate, 0.041 g of sodium chloride, 0.0072 g ofsodium bromide, 0.0506 g of sodium hydroxide, and 0.0416 g of aluminumnitrate nonahydrate were added to and dissolved in 540 g of ethyleneglycol to prepare a solution X. In a container different from the above,4.25 g of silver nitrate was added to and dissolved in 20 g of ethyleneglycol to prepare a solution Y.

Under an air atmosphere, the whole volume of the solution X was heatedfrom normal temperature to 115° C. with stirring, and then the wholevolume of the solution Y was added to the solution X over 1 minute.After completing the addition of the solution Y, the solution wasmaintained at 115° C. for 24 hours while further keeping the stirringstate. Then, the reaction liquid was cooled to normal temperature. Aftercooling, the solid was washed in the same manner as in Example 1,whereby an aqueous dispersion of silver nanowires coated with thecopolymer was obtained.

In FIG. 12, an SEM photograph of the silver nanowires is illustrated.According to the result of measuring the average diameter and theaverage length in the same manner as in Example 1, the average diameterwas 45 nm, the average length was 15 μm, and the average aspect ratiowas 15000 nm/45 nm≅333.

Substitution of Silver Nanowire Coating Substance into Thiol

As a thiol for substitution on the wire surfaces,3-mercapto-1,2-propanediol having solubility in a polar solvent wasprovided.

The polymer-coated silver nanowires synthesized in the above method wereadjusted in concentration so as to give a silver concentration of 0.5%by mass. At normal temperature, 5 mL of acetone was added to 10 mL ofthe silver nanowire dispersion and the mixture was stirred for 10seconds. The resulting mixture was a liquid A. Using a containerdifferent from the above, 3-mercapto-1,2-propanediol was added to 10 mLof pure water so as to give a concentration of 10 mmol, and dissolved inthe pure water with ultrasonication. The resulting mixture was a liquidB. The liquid B was added to the liquid A, and the mixture was stirredfor 2 minutes. The liquid was transferred into a centrifugal tube, 30 mLof methanol was added thereto, and the mixture was centrifuged underconditions of a centrifugal force of 700 G and 5 minutes. After thecentrifugation, the supernatant was removed to collect a solid. After 30mL of methanol was added to the solid collected to disperse the solidtherein, the dispersion was centrifuged under conditions of acentrifugal force of 700 G and 5 minutes. This operation was repeatedtwice, and the obtained final solid was dispersed in pure water, wherebya polar solvent (aqueous) dispersion of silver nanowires was obtained.

In FIG. 13A, an SEM photograph of the polymer-coated silver nanowiresbefore the coating substance substitution treatment (the silvernanowires of FIG. 12 above observed under the same conditions as in FIG.13B below) is illustrated. In FIG. 13B, an SEM photograph of thethiol-coated silver nanowires after the coating substance substitutiontreatment is illustrated.

In each of the metal nanowires before the coating substance substitutionand those after the substitution, the liquid component was volatilizedin a drying oven to obtain a dry sample. Using a TG-DTA apparatus(manufactured by Rigaku Corporation, Thermo Plus TG-8120), 10 mg of thedry sample was heated from a normal temperature to 750° C. at atemperature rising rate of 5° C./min, and the weight of the sample ateach temperature was measured every 1 second, thereby obtaining a TGcurve.

In FIG. 14, TG curves of the dry samples of silver nanowires before andafter the coating substance substitution are illustrated. When theignition losses until T=750° C. were compared, with an ignition lossuntil T° C. being represented by ((sample weight at temperatureT)−(sample weight before temperature rising))/(sample weight beforetemperature rising)×100, the polymer (the copolymer)-coated silvernanowires before the coating substance substitution had a value of15.1%, and the thiol (3-mercapto-1,2-propanediol)-coated silvernanowires after the coating substance substitution had a value of 3.7%.Also in silver nanowires coated with an organic protective agent(surfactant) having good dispersibility in a polar solvent such aswater, silver nanowires coated not with a polymer having strongmultiple-point adsorptivity but with a thiol which is considered to beadsorbed at one point were obtained.

Example 5

A polar solvent (aqueous) dispersion of silver nanowires was obtained inthe same manner as in Example 4 except that in the substitutionoperation of silver nanowire coating substance into a thiol,monoethanolamine thioglycolate was used as the thiol for substitution onthe wire surfaces. As the silver nanowires subjected to the substitutionoperation, the same nanowires as in Example 4 (the copolymer-coatedsilver nanowires described above) were used. According to a TG curveobtained under the same conditions as in Example 4, the ignition lossuntil 750° C. of the thiol (3-mercapto-1,2-propanediol)-coated silvernanowires after the coating substance substitution obtained in thisexample was 2.9%.

Example 6

A polar solvent (aqueous) dispersion of silver nanowires was obtained inthe same manner as in Example 4 except that in the substitutionoperation of silver nanowire coating substance into a thiol, ammoniumthioglycolate was used as the thiol for substitution on the wiresurfaces. As the silver nanowires subjected to the substitutionoperation, the same nanowires as in Example 4 (the copolymer-coatedsilver nanowires described above) were used. According to a TG curveobtained under the same conditions as in Example 4, the ignition lossuntil 750° C. of the thiol (ammonium thioglycolate)-coated silvernanowires after the coating substance substitution obtained in thisexample was 3.5%.

Example 7

A polar solvent (aqueous) dispersion of silver nanowires were obtainedin the same manner as in Example 4 except that in the substitutionoperation of silver nanowire coating substance into a thiol, thiomalicacid was used as the thiol for the substitution on wire surfaces. As thesilver nanowires subjected to the substitution operation, the samenanowires as in Example 4 (the copolymer-coated silver nanowiresdescribed above) were used. According to a TG curve obtained under thesame conditions as in Example 4, the ignition loss until 750° C. of thethiol (thiomalic acid)-coated silver nanowires after the coatingsubstance substitution obtained in this example was 5.5%.

Comparative Example 1 Synthesis of Silver Nanowires

As an organic protective agent, the same copolymer of vinylpyrrolidoneand diallyldimethylammonium nitrate as in Example 4 was provided.

At a normal temperature, 83.87 g of the copolymer of vinylpyrrolidoneand diallyldimethylammonium nitrate, 0.48 g of lithium chloride, 0.13 gof potassium bromide, 0.48 g of lithium hydroxide, 3.33 g of a1,2-propanediol solution having an aluminum nitrate nonahydrate contentof 20% by mass were added to and dissolved in 7800 g of propylene glycol(1,2-propanediol) to prepare a solution X. In a container different fromthe above, 67.96 g of silver nitrate was added to and dissolved in 320 gof 1,2-propanediol to prepare a solution Y.

The whole volume of the solution X was heated from normal temperature to115° C., and then stirred at 175 rpm for 20 minutes. After 20 minutes ofthe stirring, into the solution X at 115° C., the solution Y was addedover 30 seconds with a tube pump. The solution was maintained at 115° C.for 12 hours while further keeping the stirring state, thereby obtaininga reaction liquid in which precipitation reaction of silver wascompleted. Then, washing was performed in the same manner as in Example1, whereby an aqueous dispersion of silver nanowires coated with thecopolymer was obtained.

Attempt of Substitution of Silver Nanowire Coating Substance

Malonic acid was provided as an organic substance for attemptingsubstitution on wire surfaces.

The polymer-coated silver nanowires synthesized by the above method wereadjusted in concentration so as to give a silver concentration of 0.2%by mass. At normal temperature, 10 mmol of malonic acid was added to 20mL of the silver nanowire dispersion, and the mixture was stirred at 40°C. for 8 hours. The liquid after stirring was transferred into a centfugal tube, 150 mL of pure water was added thereto, and the mixture wascentrifuged under conditions of a centrifugal force of 1860 g and 15minutes. After the centrifugation, the supernatant was removed tocollect a solid. To the solid collected, 150 mL of pure water was added,and the mixture was centrifuged under conditions of a centrifugal forceof 1860 G and 15 minutes. This operation was repeated twice, and theobtained final solid was dispersed in pure water, whereby a polarsolvent (aqueous) dispersion of silver nanowires was obtained.

In each of the metal nanowires before the coating substance substitutionattempt and those after the substitution attempt, the liquid componentwas volatilized in a drying oven to obtain a dry sample. Using a TG-DTAapparatus (manufactured by SII, TG/DTA6300), 10 mg of the dry sample washeated from a normal temperature to 700° C. at a temperature rising rateof 10° C./min, and the weight of the sample at each temperature wasmeasured every 1 second, thereby obtaining a TG curve. According to theresult, the ignition loss until 700° C. of a polymer (thecopolymer)-coated silver nanowires before the coating substancesubstitution attempt used in this example was 8.1%, and the ignitionloss until 700° C. of the silver nanowires after the coating substancesubstitution attempt was 12.3%. Since the ignition loss was increasedrelative to the case of the original polymer, it is considered that, inspite of the stirring for a prolonged time of 8 hours, substitutionreaction from the polymer to a carboxylic acid did not proceed. In otherwords, it is considered that a carboxylic acid having no thiol groupdoes not have adsorptivity to silver that is strong enough to desorb apolymer adhered to metal silver surfaces of wires.

Comparative Example 2

An experiment was performed in the same manner as in Comparative Example1 except that in the substitution attempt operation of silver nanowirecoating substance, malic acid was used as an organic substance forattempting substitution on the wire surfaces. As the silver nanowiressubjected to the substitution attempt operation, the same nanowires asin Comparative Example 1 (the copolymer-coated silver nanowiresdescribed above) were used. According to a TG curve obtained in the sameconditions as in Comparative Example 1, the ignition loss until 700° C.of the silver nanowires after the coating substance substitution attemptwas 9.7%. Also in this example, the ignition loss was increased relativeto the state with the original polymer, and it is considered that thesubstitution reaction from the polymer to the carboxylic acid did notproceed.

Comparative Example 3

An experiment was performed in the same manner as in Comparative Example1 except that in the substitution attempt operation of silver nanowirecoating substance, ethylene diamine was used as an organic substance forattempting substitution on the wire surfaces. As the silver nanowiressubjected to the substitution attempt operation, the same nanowires asin Comparative Example 1 (the copolymer-coated silver nanowiresdescribed above) were used. According to a TG curve obtained in the sameconditions as in Comparative Example 1, the ignition loss until 700° C.of the silver nanowires after the coating substance substitution attemptwas 13.0%. Also in this example, the ignition loss was increasedrelative to the state with the original polymer, and it is consideredthat the substitution reaction from the polymer to the amine did notproceed. In other words, it is considered that an amine having no thiolgroup does not have adsorptivity to silver that is strong enough todesorb a polymer adhered to metal silver surfaces of wires.

Comparative Example 4

An experiment was performed in the same manner as in Comparative Example1 except that in the substitution attempt operation of silver nanowirecoating substance, propane diamine was used as an organic substance forattempting substitution on the wire surfaces. As the silver nanowiressubjected to the substitution attempt operation, the same nanowires asin Comparative Example 1 (the copolymer-coated silver nanowiresdescribed above) were used. According to a TG curve obtained under thesame conditions as in Comparative Example 1, the ignition loss until700° C. of the silver nanowires after the coating substance substitutionattempt was 11.8%. Also in this example, the ignition loss was increasedrelative to the state with the original polymer, and it is consideredthat the substitution reaction from the polymer to the amine did notproceed.

What is claimed is:
 1. Silver nanowires having an average diameter of100 nm or less and an average length of 5 μm or more, wherein a thiolhaving a molecular weight of 75 to 300 is adhered on surfaces of metalsilver.
 2. The silver nanowires according to claim 1, wherein the thiolhas only one thiol group in a molecule.
 3. The silver nanowiresaccording to claim 1, wherein the thiol is one or more selected from thegroup consisting of 1-dodecanethiol, 1-decanethiol, and 1-octanethiol.4. The silver nanowires according to claim 1, wherein the thiol is oneor more selected from the group consisting of3-mercapto-1,2-propanediol, monoethanolamine thioglycolate, ammoniumthioglycolate, and thiomalic acid.
 5. A method for producing silvernanowires comprising mixing a liquid A in which silver nanowires coatedwith a polymer are dispersed and a liquid B in which a thiol having amolecular weight of 75 to 300 is dissolved to thereby substitute anadhering substance on metal silver surfaces from the polymer to thethiol.
 6. The method for producing silver nanowires according to claim5, wherein the silver nanowires dispersed in the liquid A have anaverage diameter of 100 nm or less and an average length of 5 μm ormore.
 7. The method for producing silver nanowires according to claim 5,wherein the thiol has only one thiol group in a molecule.
 8. The methodfor producing silver nanowires according to claim 5, wherein the thiolis one or more selected from the group consisting of 1-dodecanethiol,l-decanethiol, and 1-octanethiol.
 9. The method for producing silvernanowires according to claim 5, wherein the thiol is one or moreselected from the group consisting of 3-mercapto-1,2-propanediol,monoethanolamine thioglycolate, ammonium thioglycolate, and thiomalicacid.
 10. The method for producing silver nanowires according to claim5, wherein the polymer is polyvinylpyrrolidone (PVP).
 11. The method forproducing silver nanowires according to claim 5, wherein the polymer isa copolymer having a vinylpyrrolidone constituting unit.
 12. The methodfor producing silver nanowires according to claim 5, wherein the liquidA and the liquid B are mixed in the presence of one or two of acetoneand isopropanol.
 13. A silver nanowire dispersion, wherein silvernanowires as set forth in claim 1 are dispersed in a liquid medium.